Use of rare earth metal salt solutions for sealing of anodized aluminum for corrosion protection and paint adhesion

ABSTRACT

A process for sealing the surface coating formed by anodizing an aluminum or aluminum alloy substrate (for example, aerospace, commercial, and architectural products), the process including the steps of:  
     (a) providing an aluminum or aluminum alloy substrate with a surface coating formed thereon by anodizing the aluminum or aluminum alloy substrate;  
     (b) providing a sealing solution comprising a dilute solution of a rare earth metal salt selected from the group consisting of cerium salts and yttrium salts; and  
     (c) contacting the substrate with the sealing solution for a sufficient amount of time to seal the surface coating on the substrate.  
     Also disclosed is a chemical sealing solution for sealing the surface coating formed by anodizing an aluminum or aluminum alloy substrate, the solution being a dilute solution of a rare earth metal salt selected from the group consisting of cerium salts and yttrium salts.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. provisionalapplication No. 60/046,234 filed on May 12, 1997.

BACKGROUND OF THE INVENTION

[0002] This environmental-quality invention is in the field of sealingthe surface coatings produced by anodizing aluminum and aluminum alloysubstrates (for example, aerospace, commercial, and architecturalproducts). The invention produces sealed anodization coatings exhibitinggood corrosion resistance performance while maintaining acceptablelevels of paint adhesion performance.

[0003] The International Agency for Research on Cancer has identifiedboth chromium and nickel compounds along with many other pollutants asconfirmed human carcinogens. The Boeing Company (Boeing), along withmany other companies, has voluntarily agreed with the U.S. EnvironmentalProtection Agency (EPA) to reduce the use of the seventeen mosthazardous pollutants which include these compounds. Currently, the onlyapproved sealing solution for the coating produced by the boricacid-sulfuric acid anodizing process is a dilute (45-75 ppm) chromateseal solution. The purpose of the chromate sealing solution is tohydrate surface oxide while entrapping the hexavalent chromium. Thehexavalent chromium acts as a corrosion inhibitor to further enhance thecorrosion resistance of the anodized coating. Using this dilute chromateseal solution, production operations can use the boric acid-sulfuricacid anodizing process on aluminum alloys 2024, 6061, and 7075 andproduce parts that pass a two-week salt spray test and meet therequirements for paint adhesion. Unfortunately, the dilute chromatesealing solution is a hazardous pollutant.

[0004] The unsealed aluminum oxide produced by anodizing is usuallymodeled as two oxide layers on an aluminum substrate. The inner layer isa thin continuous barrier layer of less than 500 angstroms thickness.The outer layer is a discontinuous coating with pores that may penetratefrom the outside surface to the barrier layer. These pores are thesource of potential corrosion pitting problems that occur in salt sprayand other atmospheric environments. In the dilute chromate seal solutionprocess, these aluminum oxide pores are hydrated with entrappedhexavalent chromium. This filling of the pores enhances the corrosionprotection of the anodized coating on the aluminum substrate.

[0005] In B. Yaffe, Metal Finishing, May 1990, vol. 41 (1990), theauthor reviews the known methods of sealing anodized aluminum, such assealing in steam and hot water, nickel acetate, dichromate, and variouscold sealing methods. Some of the newer sealing methods have beendeveloped due to environmental concerns and the desire to lower costs.Cold sealing in nickel fluoride has been introduced to lower thesecosts. However, health hazards have been observed recently for nickelsalts, which can cause allergic contact dermatitis. In NASA Tech Briefs,May 1995, a sulfuric acid anodizing process with a lower temperaturenickel acetate seal is described. This process produces thin anodizedlayers that are not detrimental to the fatigue properties of thealuminum substrate, but does not address the health hazards due to theuse of nickel salts. In Boeing's boric acid-sulfuric acid anodizingprocess, anodized layers of about 1 μm thickness are produced, which arethen sealed using a dilute chromate solution (as described in BoeingProcess Specification BAC 5632, “Boric Acid-Sulfuric Acid Anodizing”).

[0006] In a study to develop an overall corrosion protection system foraluminum alloys, co-inventor Mansfeld developed a treatment forcommercial aluminum alloys using two rare earth metal salt solutionsthat produced surfaces with excellent resistance to pitting (seeMansfeld et al. U.S. Pat. No. 5,194,138, “Method For Creating ACorrosion-Resistant Aluminum Surface”). For commercial aluminum alloyshaving a high copper content, co-inventor Mansfeld developed anadditional pre-treatment to remove copper from the outer surface tofurther enhance corrosion protection (see Mansfeld et al. U.S. Pat. No.5,582,654, “Method For Creating A Corrosion-Resistant Surface OnAluminum Alloys Having A High Copper Content”).

BRIEF SUMMARY OF THE INVENTION

[0007] In one aspect, the invention is a process for sealing the surfacecoating formed by anodizing an aluminum or aluminum alloy substrate (forexample, aerospace, commercial, and architectural products), the processincluding the steps of:

[0008] (a) providing an aluminum or aluminum alloy substrate with asurface coating formed thereon by anodizing the aluminum or aluminumalloy substrate;

[0009] (b) providing a sealing solution comprising a dilute solution ofa rare earth metal salt selected from the group consisting of ceriumsalts and yttrium salts; and

[0010] (c) contacting the substrate with the sealing solution for asufficient amount of time to seal the surface coating on the substrate.

[0011] In another aspect, the invention is a chemical sealing solutionfor sealing the surface coating formed by anodizing an aluminum oraluminum alloy substrate, the solution being a dilute solution of a rareearth metal salt selected from the group consisting of cerium salts andyttrium salts.

DETAILED DESCRIPTION OF THE INVENTION

[0012] The rare earth metal salt sealing solutions described hereinprovide an alternative to the commonly-used chromate-type seal solutionsfor the boric-sulfuric acid anodizing process, for the sulfuric acidanodizing process, and for the chromic acid anodizing process. Theserare earth metal salt sealing solutions contain low toxicity materialsthat may be disposed of easily.

[0013] Aluminum alloys anodized by the boric acid-sulfuric acidanodizing process and then sealed with a rare earth metal salt sealingsolution meet the same performance requirements called out for thesealloys when sealed using a dilute chromate seal solution. These testsinclude the salt-spray test conducted in accordance with ASTM B117(“Standard Test Method of Salt Spray (Fog) Testing”) and the paintadhesion test conducted in accordance with Boeing Support Standard BSS7225 (“Adhesion, Tape Test”). Test panels of aluminum alloys 6061 and7075 passed the 336-hour salt spray test with less than one pit per 10sq. in., which is the passing criterion. Test panels of aluminum alloy2024 require further optimization since they had about two pits per 10sq. in. Test panels of anodized aluminum alloys 2024, 6061, and 7075sealed with rare earth metal salt sealing solutions and then sprayedwith a paint qualified under Boeing Material Standard BMS 10-11(“Chemical and Solvent Resistant Finish”) passed the dry adhesion,24-hour wet adhesion, and seven-day adhesion tests. There was no primerlift off from any panel in any of the three adhesion tests although upto {fraction (1/32)} in. primer lift off beyond the scribe isacceptable.

[0014] The objective of this invention is to replace the current dilutechromate sealing solution with an equivalent-performing or betternon-chromate seal solution using either a similar or an alternativeinhibitive approach and chemical substances that are not currently orforeseen to be listed as toxic by the EPA. Also, our objective is tominimize upset to the current boric acid-sulfuric acid anodizing processby providing a seal whereby parts need not be sorted due to alloycomposition.

[0015] We conducted research to evaluate rare earth metal salt sealingsolutions such as cerium salts, yttrium salts, and others as areplacement to the currently successful dilute chromate seal solutionused for sealing the coatings produced by the boric acid-sulfuric acidanodizing process. More specifically, we included: yttrium acetate,yttrium sulfate, yttrium chloride, cerium nitrate, cerium acetate,cerium sulfate, nickel fluoride (a European standard), boiling water,and dilute chromate seal solution as our standard. The aluminum alloysincluded 2024, 6061 and 7075. The test methods included electrochemicalimpedance spectroscopy (EIS) and optical microscopy examination at 30×of the panels after immersion in 0.5N NaCl solutions.

Sealing Process

[0016] The sealing process for an anodized aluminum alloy part is asfollows:

[0017] Sealing:

[0018] Immerse parts in the sealing solution at the specifiedtemperature for the prescribed period of time.

EXAMPLE 1

[0019] Panels (4 in.×6 in.) of aluminum alloys 2024, 6061, 7075 werecoated in accordance with the boric acid-sulfuric acid anodizing processas described in Boeing Process Specification BAC 5632, “BoricAcid-Sulfuric Acid Anodizing”. Then a 50 mM cerium nitrate sealingsolution (mM is the abbreviation for millimolar) was prepared bydissolving the cerium nitrate salt in distilled water and adjusting topH 6 using nitric acid at room temperature. The solution was heated tothe boiling temperature which is approximately 100° C. Panels wereimmersed in the sealing solution for 30 minutes.

EXAMPLE 2

[0020] Panels (4 in.×6 in.) of aluminum alloys 2024, 6061, 7075 werecoated in accordance with the boric acid-sulfuric acid anodizing processas described in Boeing Process Specification BAC 5632, “BoricAcid-Sulfuric Acid Anodizing”.

[0021] Then a 50 mM yttrium sulfate sealing solution was prepared bydissolving the yttrium sulfate salt in distilled water and adjusting topH 6 using nitric acid at room temperature. The solution was heated tothe boiling temperature which is approximately 100° C. Panels wereimmersed in the sealing solution for 30 minutes.

EXAMPLE 3

[0022] Panels (4 in.×6 in.) of aluminum alloys 2024, 6061, 7075 werecoated in accordance with the boric acid-sulfuric acid anodizing processas described in Boeing Process Specification BAC 5632, “BoricAcid-Sulfuric Acid Anodizing”. Then a 50 mM cerium sulfate sealingsolution was prepared by dissolving the cerium sulfate salt in distilledwater and adjusting to pH 5.5 using nitric acid at room temperature. Thesolution was heated to the boiling temperature which is approximately100° C. Panels were immersed in the sealing solution for 15 minutes.

EXAMPLE 4

[0023] Panels (3 in.×3 in.) of aluminum alloys 2024 and 6061 were coatedin 15 wt. pct. sulfuric acid. Then a saturated cerium acetate sealingsolution was prepared by dissolving cerium acetate salt in distilledwater as described by Mansfeld et al., Plating and Metal Finishing, Dec.1997, vol. 84 (1997). The solution was heated to boiling temperaturewhich is approximately 100° C. Panels were immersed in the sealingsolution for 40 minutes. After sealing, the panels were rinsed withdeionized water and air dried.

EXAMPLE 5

[0024] Panels (3 in.×3 in.) of aluminum alloy 6061 were coated in 15 wt.pct. -sulfuric acid. Then a saturated cerium acetate sealing solutionwas prepared by dissolving cerium acetate salt in distilled water asdescribed by Mansfeld et al., Plating and Metal Finishing, Dec. 1997,vol. 84 (1997). The solution was heated to approximately 80-85° C.Panels were immersed in the sealing solution for 40 minutes. Aftersealing, the panels were rinsed with deionized water and air dried.

EXAMPLE 6

[0025] Panels (3 in.×3 in.) of aluminum alloy 7075 were coated in 15 wt.pct. sulfuric acid. Then a saturated cerium acetate sealing solution wasprepared by dissolving cerium acetate salt in distilled water asdescribed by Mansfeld et al., Plating and Metal Finishing, Dec. 1997,vol. 84 (1997). The solution was heated to approximately 80-85° C.Panels were immersed in the sealing solution for 20 minutes. Aftersealing, the panels were rinsed with deionized water and air dried.

EXAMPLE 7

[0026] Panels (3 in.×3 in.) of aluminum alloy 2024 were coated in 15 wt.pct. sulfuric acid. Then a saturated cerium acetate sealing solution wasprepared by dissolving cerium acetate salt in distilled water asdescribed by Mansfeld et al., Plating and Metal Finishing, Dec. 1997,vol. 84 (1997). The solution was heated to approximately 80-85° C.Panels were immersed in the sealing solution for 20 minutes. Aftersealing, the panels were rinsed with deionized water and air dried.

[0027] Rinsing:

[0028] Remove parts from the sealing solution, water immersion rinse at50° C. for five minutes, followed by subsequent rinse at roomtemperature for five minutes.

[0029] Drying:

[0030] Dry sample with dry oil-free air.

Chemical Concentration, pH, Temperature, And Immersion Time

[0031] The chemical concentration of the dissolved rare earth metal saltin the sealing solution may be from about 10 mM to about 350 mM. The pHof the sealing solution may be from about 3.0 to about 9.0. Thetemperature of the sealing solution may be from about 60° C. to theboiling temperature of the sealing solution. The immersion time in thesealing solution may be from about 10 minutes to about 60 minutes.

Results of Electrochemical Impedance Spectroscopy (EIS) and OpticalMicroscopy at 30×

[0032] Numerous electrochemical impedance spectroscopy (EIS) runs wereperformed to generate Bode plots (logarithm impedance versus logarithmfrequency; phase angle versus logarithm frequency) that include work onpanels of sealed and unsealed coatings made by the boric acid-sulfuricacid anodizing process. At the end of testing, the panels were examinedat 30× magnification to determine the number of pits and to size thepits as either small or large. From these data, we selected yttriumsulfate, cerium nitrate, and cerium sulfate sealing solutions as themore promising candidates. The selected rare earth metal salt sealingsolutions were evaluated in corrosion and adhesion testing.

Results of Corrosion Testing

[0033] Duplicate 4 in.×6 in. salt spray panels of alloys 2024, 6061, and7075 with a coating produced by the boric acid-sulfuric acid anodizingprocess were sealed with cerium nitrate, yttrium sulfate, and ceriumsulfate, as in the above sealing process Examples 1, 2, and 3,respectively. After 336 hours of salt spray testing, the panels werevisually examined. The passing criterion is that there shall be no morethan five pits on a 3 in.×10 in. panel or more than nine pits in 90square inches of test area. The pit density shall not exceed one pit per10 sq. in. All alloy 6061 and alloy 7075 panels had one or no pits onthe 24 sq. in. surface. The alloy 2024 panels with yttrium sulfate andcerium nitrate seal had about five pits per panel, which is about twopits per 10 sq. in. The alloy 2024 panel with cerium sulfate hadmultiple pits.

Results of Paint Adhesion Testing

[0034] Panels of alloys 2024, 6061, and 7075 with a coating produced bythe boric acid-sulfuric acid anodizing process were sealed with yttriumsulfate, cerium nitrate, and cerium sulfate, as in the above sealingprocess Examples 1, 2, and 3, respectively. Each panel was sprayed withone coat of a paint (manufactured by Deft) qualified under BoeingMaterial Specification BMS 10-11, Grade E, and allowed to cure at roomtemperature for seven days. Testing included: dry adhesion, 24 hour wetadhesion, and 7 day wet adhesion. The passing criterion in the scribearea is that there shall be no paint lift off {fraction (1/32)} in.beyond the scribe after the tape adhesion test. The test results showedno paint lift off from any panel. The three alloys each sealed with thethree different seal solutions all passed the paint adhesion test.

[0035] The patents, specifications, and other publications referencedabove are incorporated herein by reference.

[0036] As will be apparent to those skilled in the art to which theinvention is addressed, the present invention may be embodied in formsother than those specifically disclosed above, without departing fromthe spirit or essential characteristics of the invention. The particularembodiments of the invention described above and the particular detailsof the processes described are therefore to be considered in allrespects as illustrative and not restrictive. The scope of the presentinvention is as set forth in the appended claims rather than beinglimited to the examples set forth in the foregoing description. Any andall equivalents are intended to be embraced by the claims.

What is claimed is:
 1. A process for sealing the surface coating formedby anodizing an aluminum or aluminum alloy substrate, said processcomprising the steps of: (a) providing an aluminum or aluminum alloysubstrate with a surface coating formed thereon by anodizing saidaluminum or aluminum alloy substrate; (b) providing a sealing solutioncomprising a dilute solution of a rare earth metal salt selected fromthe group consisting of cerium salts and yttrium salts; and (c)contacting said surface coating on said substrate with said sealingsolution for a sufficient amount of time to seal said surface coating onsaid substrate.
 2. The process of claim 1 wherein said sealing solutioncomprises a dilute solution of cerium salts.
 3. The process of claim 1wherein said sealing solution comprises a dilute solution of yttriumsalts.
 4. The process of claim 1 wherein said sealing solution comprisesa dilute solution of a rare earth metal salt selected from the groupconsisting of cerium nitrate, yttrium sulfate, and cerium sulfate. 5.The process of claim 1 wherein the chemical concentration of thedissolved rare earth metal salt in the sealing solution is from about 10mM to about 350 mM.
 6. The process of claim 1 wherein the pH of saidsealing solution is from about 3.0 to about 9.0.
 7. The process of claim1 wherein the immersion time in said sealing solution is from about 10minutes to about 60 minutes.
 8. The process of claim 1 wherein thetemperature of the sealing solution is from about 60° C. to the boilingtemperature of the sealing solution.
 9. A chemical sealing solution forsealing the surface coating formed by anodizing an aluminum or aluminumalloy substrate, said solution comprising a dilute solution of a rareearth metal salt selected from the group consisting of cerium salts andyttrium salts.
 10. The chemical sealing solution of claim 9 wherein saidsealing solution comprises a dilute solution of cerium salts.
 11. Thechemical sealing solution of claim 9 wherein said sealing solutioncomprises a dilute solution of yttrium salts.
 12. The chemical sealingsolution of claim 9 wherein said sealing solution comprises a dilutesolution of a rare earth metal salt selected from the group consistingof cerium nitrate, yttrium sulfate, and cerium sulfate.
 13. The chemicalsealing solution of claim 9 wherein the chemical concentration of thedissolved rare earth metal salt in said sealing solution is from about10 mM to about 350 mM.
 14. The chemical sealing solution of claim 9wherein the pH of said sealing solution is from about 3.0 to about 9.0.15. The chemical sealing solution of claim 9 wherein the temperature ofsaid sealing solution is from about 60° C. to the boiling temperature ofthe sealing solution.